1. Field of the Invention
The present invention is generally in the field of electrical circuits. More particularly, the invention is in the field of electrical circuits used in communication devices.
2. Background Art
In-phase (I) and quadrature (Q) signals are typically utilized in modulation and demodulation sections of transceivers in cellular handsets and other types of communication devices. The I and Q signals, which are 90 degrees out of phase, can be generated, for example, by coupling an input local oscillator signal to first and second outputs via different RC networks. For example, one RC network can include a capacitor coupled between the input and the first output and a resistor coupled between the first output and ground and the other RC network can include a capacitor coupled between the input and the second output and a resistor coupled between the second output and ground. To achieve balanced I and Q signals (i.e. I and Q signals having the same amplitude), the resistors in each RC network and the capacitors in each RC network must have the same and predetermined value according to the operation frequency. However, process variations, particularly in the resistors, can cause the I and Q signals to significantly differ in amplitude, thereby undesirably affecting transceiver performance.
In one approach, system level calibration can be used to reduce the difference in amplitude between the I and Q signals. However, system level calibration is not effective if the difference in amplitude of the I and Q signals is too great. In another approach, calibrated resistors can be used in the RC networks to achieve I and Q signals having similar amplitudes. However, at high local oscillator frequencies, such as 5.0 GHz, each RC network requires a very small value resistor, which is difficult to calibrate.